Light emitting device and display apparatus and read apparatus using the light emitting device

ABSTRACT

A light emitting device provided with a plurality of types of light sources having different light emitting colors and with a light emission control means for allowing light to emit, during a specified period of monitoring a light emitting intensity, from at least one light source out of the plurality of types of light sources at a light emitting intensity different from that available outside the specified period. Accordingly, when a plurality of types of light sources are used, the light emitting intensities of a plurality of types of light sources can be monitored with light sensors of types fewer than the types of light sources to control while points and a brightness characteristics.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a light-emitting device comprising alight source which emits light having a plurality of colors, a displayapparatus using the light-emitting device, and a read apparatus usingthe light-emitting device.

2. Description of the Related Art

It has been conventionally known that, in some types of transmissiveliquid crystal which employ a backlight including a side light, andreflective liquid crystals which employ a front light, a light-emittingdevice, which includes a white cold cathode fluorescent tube or a whitelight-emitting diode (LED) as a light source, is mounted as a back lightor a front light for display. Particularly, many types of cellularphones which have rapidly become popular recently-employ a white LED.

However, a light source using a white cold cathode fluorescent tube anda white LED have a problem that white point and luminancecharacteristics vary largely depending on changes in temperaturecharacteristics and changes over time. In order to solve this problem,the following two methods have been proposed, for example.

The first method is effective in the case where multiple types of lightsources emitting light of different colors are switched by atime-division to provide a white light source. As described in JapaneseLaid-Open Publication No. 10-49074, for example, light sources ofrespective colors are monitored by an optical sensor and changes inamounts of light are fed back to respective light sources for emittingwhite light.

The second method is effective for the case where multiple types oflight sources emitting light of different colors are made to emit lightat the same time to provide a white light source. As described inJapanese Laid-Open Publication No. 11-295689, light sources ofrespective colors are monitored by an optical sensor and changes inamounts of light are fed back to respective light sources so as to havean equal value as a certain predetermined value for emitting whitelight.

General examples of light-emitting operations of light sources forallowing the multiple types of light sources to emit light at the sametime and the colors of emitted light to be mixed for providing whitecolor in the second method mentioned above are shown in FIGS. 12 and 13.The multiple types of the light sources are, for example, a red LED, agreen LED, and a blue LED Methods for controlling a light-emittingoperation of the light sources are roughly divided into two types: apulse width control method shown in FIG. 12; and a current value controlmethod shown in FIG. 13. A method which combines these two methods isalso possible.

FIGS. 12( a), (b) and (a) are graphs which respectively show theperformance of pulse width control of current values flowing through thered, green and blue light sources, with the horizontal axes indicatingtime and the vertical axes indicating current value. By performing pulsewidth control of the emission intensities of the light sources, i.e., bycontrolling the time lengths of the light emitted by the light sourceswhile the emission intensities of the light sources are maintainedconstant, apparent light emission intensities change. For example, inorder to increase the apparent light emission intensities, the lightemitting time of the light sources is lengthened. In order to reduce theapparent emission Intensities, the light emitting time of the lightsources is shortened. In this way, the apparent light intensities of thelight sources are controlled by adjusting the length of time while lightis emitted and the length of time while light is not emitted.

Taking the light-emitting operation of the red light source as shown inFIG. 12( a) as a standard, the green light source as shown in FIG. 12(b) emits light for a period of time shorter than that of the red lightsource in the first cycle. In the next cycle, the green light sourceemits the light for a further shorter time to reduce the apparentemission intensities. The blue light source as shown in FIG. 12( c)emits light for a period of time longer than the red light source. Inthe next cycle, the blue light source emits light for further longertime to increase the apparent emission intensities.

As described above, in the pulse width control method, thelight-emitting time of the light sources are controlled at apredetermined frequency while the values of the current flowing throughthe light sources are maintained constant. The frequency should be setto a cycle which is not perceived by the eyes of a human, for example,60 Hz or higher. If the frequency is set too high, the cost for thedriving circuit increases. Thus, generally the frequency is set to about200 Hz.

Similarly to FIG. 12. FIGS. 13( a), (b) and (a) are graphs whichrespectively show sequentially changing current values flowing throughthe red, green and blue light sources, with the horizontal axesindicating the time and the vertical axes indicating the current values.In this case, by sequentially changing the amount of the current flowingthrough the light sources over time, the emission intensities of thelight sources is controlled. In order to increase the emissionintensities, the current value is increased. In order to reduce theemission intensities, the current value is reduced. For example, in thered light source as shown in FIG. 13( a), the emission intensity isincreased by increasing the current values flowing through the red lightsource. In the green light source as shown in FIG. 13( b), the emissionintensity is reduced by reducing the current values. As shown in FIG.13( c), the emission intensity may be maintained constant by allowing acurrent which is constant in terms of time to flow.

The first and the second methods described above have the followingproblems. First, the time-division switching method described inJapanese Laid-Open Publication No. 10-49074 has an advantage that theemission intensities of the light sources can be monitored by a singletype optical sensor, but the method has a critical problem that it iseffective for only the time-division method, in which light sources areturned on one type at a time in turn, and it cannot be applied to amethod other than the time-division method.

Further, the simultaneous light-emitting method described in JapaneseLaid-Open Publication No. 11-295689 has a problem that the cost is highbecause a color separation filter is necessary in addition to threetypes of optical sensor corresponding to the red, green, and blue lightsources, and a problem that control of the emission intensities becomesinaccurate due to a variance in optical sensor outputs because threetypes of optical sensor cannot be located at the same place.

Further, although it is desirable that the backlight emits lightuniformly across its entire surface, it is difficult to actually emitlight in a uniform manner. Thus, uneven luminance is usually generated.It is also a concern that, when three types of the light sources, i.e.,a red light source, a green light source, and a blue light source areused instead of a light source emitting white light, uneven color may begenerated because the colors of the light from the light sources are notperfectly mixed. In the case where such uneven luminance or uneven coloris generated, variance may be a problem depending on where the displayapparatus is located.

DISCLOSURE OF THE INVENTION

The present invention has been proposed in view of various problems asdescribed above. The objective of the present invention is to provide alight-emitting device which can monitor emission intensities of multipletypes of the light sources with fewer types of optical sensors, and cancontrol white point and/or luminance properties, and a display apparatusand a read apparatus using the light-emitting device.

In order to achieve the above described objective, the present inventionprovides a light emitting device comprising: multiple types of lightsources emitting light of different colors; a light detection means formonitoring emission intensity of at least one light source among themultiple types of light sources; and a light emission control meanswhich performs control to provide a light emitting period in which allof the multiple types of the light sources emit light at the same timeat predetermined emission intensities and a monitoring period in whichat least one of the multiple types of the light sources emits light atan emission intensity different from that in the light emitting periodin which all of the multiple types of the light sources emit light atthe same time, wherein the light emission control means controls theemission intensity of the at least one light source among the multipletypes of the light sources based on emission intensity information fromthe light detection means in the monitoring period to adjust compositelight from the multiple types of the light sources to have a desiredluminance or chromaticity.

Preferably, in the present invention the light emission control means ischaracterized by turning off at least one light source among themultiple types of the light sources in the monitoring period.

Preferably, in the present invention the light emission control means ischaracterized by shifting either timing to turn on each light source ortiming to turn off each light source.

Preferably, in the present invention the light emission control means ischaracterized by decreasing the emission intensity of at least one lightsource among the multiple types of the light sources in the monitoringperiod.

Preferably, in the present invention the light emission control means ischaracterized by shifting either timing to make the emission intensityof each light source to the predetermined emission intensity or timingto decrease the emission intensity.

Preferably, in the present invention the light emission control means ischaracterized by increasing the emission intensity of at least one lightsource among the multiple types of the light sources in the monitoringperiod.

Preferably, in the present invention the light emission control means ischaracterized by shifting either timing to make the emission intensityof each light source to the predetermined emission intensity or timingto increase the emission intensity.

Preferably, in the present invention, the light detection means ischaracterized by having spectral sensitivity characteristicsapproximately matching luminosity factor characteristics with the lightemission wavelength of the at least one light source among the multipletypes of light sources.

Preferably, in the present invention, the light detection means ischaracterized by comprising a luminosity factor filter for blockinginfrared radiation.

Preferably, the present invention is characterized in that a period inwhich all of the multiple types of the light sources are turned off isprovided, and the Light detection means monitor amount of light in astate that all of the multiple types of the light sources are turnedoff.

Preferably, the present invention is characterized by comprising: alight source unit including a plurality of three types of light sources;a light guide plate for uniformly irradiating a plane with light fromthe light source unit; and an optical sensor as a light detection meansprovided in the vicinity of the light guide plate.

Preferably, the present invention is characterized by comprising: afirst light source unit including a plurality of one or two types. oflight sources; a first light guide plate for uniformly irradiating aplane with light from the first light source unit; a second light sourceunit including one or two type of light sources different from the abovelight sources; a second light guide plate for uniformly irradiating aplane with light from the second light source unit and the first lightguide plate; and an optical sensor as a light detection means providedin the vicinity of the first and the second light guide plates.

Preferably, the present invention provides a display apparatus using alight emitting device as described above.

Preferably, the present invention provides a display apparatus using alight emitting device described above, wherein: when a level of aluminance signal included in an input video signal is equal to or lessthan the threshold value, the monitoring period is started.

Preferably, the present invention provides a display apparatus using alight emitting device described above, wherein: in the period in whicheach light source is turned off, a size of a drive signal of the displayapparatus is extended.

Preferably, the present invention provides a display apparatus using alight emitting device described above, wherein: when a level of aluminance signal included in an input video signal is equal to or lessthan the threshold value, the period in which the emission intensity ofeach light source is decreased is started.

Preferably, the present invention provides a display apparatus using alight emitting device described above, wherein: in the period in whichthe emission intensity of each light source is decreased, a size of adrive signal of the display apparatus is extended.

Preferably, the present invention provides a read apparatus using alight emitting device described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a first embodiment of a lightemitting device according to the present invention.

FIG. 2 is a schematic diagram of a liquid crystal display apparatususing the light emitting device of FIG. 1 as an auxiliary light source.

FIG. 3 is a schematic diagram showing a first driving example of thelight emitting device of FIG. 1 during a monitoring period.

FIG. 4 is a schematic diagram showing a second driving example of thelight emitting device of FIG. 1 during a monitoring period.

FIG. 5 is a schematic diagram showing a third driving example of thelight emitting device of FIG. 1 during a monitoring period.

FIG. 6 is a diagram schematically showing a second embodiment of a lightemitting device according to the present invention.

FIG. 7( a)-(c) is a diagram showing light emitting operations of lightsources in a first monitoring method for monitoring the light emittingdevice of FIG. 6: and FIG. 7( d) is a diagram illustrating a lightemitting operation of the entire, light source in accordance with theabove operations.

FIG. 8( a)-(c) is a diagram showing light emitting operations of lightsources in a second monitoring method for monitoring the light emittingdevice of FIG. 6; and FIG. 8( a) is a diagram illustrating a lightemitting operation of the entire light source in accordance with theabove operations.

FIG. 9( a)-(c) is a diagram showing light emitting operations of lightsources in a third monitoring method for monitoring the light emittingdevice of FIG. 6; and FIG. 9( d) is a diagram illustrating a lightemitting operation of the entire light source in accordance with theabove operations.

FIG. 10 is a diagram schematically showing a third embodiment of a lightemitting device according to the present invention.

FIG. 11( a) is a diagram schematically showing a read apparatus usingthe light emitting device of a fourth embodiment according to thepresent invention; and FIG. 11(b) is a diagram schematically showing alight emitting device used for the, read apparatus.

FIG. 12( a)-(c) is a diagram illustrating light emitting operations whenpulse control of respective light sources is performed in a conventionallight emitting device.

FIG. 13( a)-(c) is a diagram illustrating light emitting operations whencurrent control of respective light sources is performed in aconventional light emitting device.

FIG. 14 is a graph indicating luminosity factor characteristics ofhuman, spectral sensitivity characteristics of two types of opticalsensors, and light emitting wavelengths and temperature changes of redLEDs,

FIG. 15 is a graph of characteristics of a luminosity factor filter ofan optical sensor and experimentation results in stability of lightemitting luminance.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the first through fourth embodiments of the presentinvention will be described with reference to the drawings.

First Driving Example of First Embodiment

FIG. 1 schematically shows the first embodiment of the light emittingdevice according to the present invention a In the first embodiment, asthe basic components, the light emitting device 10A includes: the lightsource unit 1 in which three types of light sources emitting light ofdifferent colors are located; a color mixing part 2 which allows threedifferent types of light generated from the light source unit 1 to berecognized as white color without color unevenness; a light guide plate3 for guiding the white light mixed in the color mixing part 2 to anentire panel of the display-apparatus (FIG. 2); an optical sensor 4 as alight detection means for monitoring the intensity of light transmittedthrough the light guide plate 3; and light-emission control means 11which receives emission intensity information of the light sourcesobtained by performing light emission control of the emissionintensities of the three types of the light sources for monitoringduring a monitoring period as monitoring results from the optical sensor4, and performs light emission control of the three types of the lightsources so as to have a predetermined emission intensity based on theemission intensity information.

FIG. 2 shows a liquid crystal display apparatus 20 which uses thelight-emitting device 10A shown in FIG. 1 as a backlight or a frontlight. A liquid crystal panel 5 is located in front of (or behind) thelight guide plate 3. In other words, in the case where the liquidcrystal panel 5 is of a transmissive type, the liquid crystal panel 5 islocated in front of the light guide plate 3, i.e., on the side of theuser. In the case where the liquid crystal panel 5 is of a reflectivetype, the liquid crystal panel 5 is located behind the light guide plate3, although this case is not illustrated;

Although the components are Illustrated to be separate from each otherin FIGS. 1 and 2 for facilitating understanding, it is desirable toposition the components close to each other. Further, in FIG. 1, thedifferences in the size of the components are emphasized forfacilitating understanding, and the actual sizes of the components aredifferent to those illustrated.

In the light emitting device 10A shown in FIGS. 1 and 2, LEDs havingthree primary colors of light, i.e., red, green and blue are placed inthe light source unit 1. Light passes through the light mixing part 2and mixing is performed to obtain white light. The white light passesthrough the light guide plate 3 and is received by the optical sensor 4.The optical sensor 4 produces a detection output corresponding to thesum of the intensities of light from LEDs which have emitted light.Usually, when red, green and is blue LEDs are turned on at the sametime, white light is generated from an appropriate emission ratio of theLEDs. Since temperature characteristics in light emission efficiency duethe heat generated by the LEDs varies depending on color, the whitecolor balance of white collapses and the white point is shifted greatly.Further, a shift in the white point due to change over time may also begenerated,

Accordingly, in the light-emission control means 11 of the presentinvention, a short monitoring period is intermittently provided whilethe red, green and blue LEDS in the light source unit 1 operate at thesame time and white light is emitted. During such a monitoring period,one or two LEDs are independently turned on at different times in turn,and the rest of the LEDs are turned off. For example, during amonitoring period, the red, green and blue LEDs are pulse-driven in turnby a pulse frequency of 200 Hz, for example.

For example, it is assumed that, during the monitoring period, the red,green and blue LEDs are driven such that they emit light one type at atime in this order and such that, while one LED is turned on, the othertwo types of LEDs are turned off the time during which the two types oflight sources are turned off is 1/200 second, which is 1 cycle of afrequency for pulse-driving a LED. In the case that three types of LEDsare turned on in turn, the monitoring period is just 3/200 seconds. Suchan operation is performed by light-emission control means ALA, which isone example of the light-emission control means 11, and is shown in FIG.3. In FIG. 3, (a) indicates the emission intensity of the red LED, (b)indicates the emission intensity of the green LED, and (c) indicates theemission intensity of the blue LED. The vertical axes indicate emissionintensity and the horizontal axes indicate time.

In FIG. 3( a)-(c), during a period from time t1 to t2, all the red,green and blue LEDs are turned on. Thus, the light-emitting device 10Aemits white light. Then, a monitoring period starts at time t2. Only thered LED emits light and the green and blue LEDs are turned off. Thus,the light emitting device 10A emits red color light. After 1/200 secondhas elapsed from time t2 it becomes time t3, and the green LED is turnedon, the red LED is turned off, and the blue LED remains in the turnedoff state. After another 1/200 second has elapsed it becomes time t4,and the blue LED is turned on, the green LED is turned off, and the redLED remains in the turned off state. Then, after another 1/200 secondhas elapsed it becomes time t5, and the monitoring period ends. Threetypes of LEDs are all turned on and the light emitting device 10Aprovides white light.

The emission intensities of the LEDs in the light source unit 1 aremonitored by optical sensor 4 only during the monitoring period t2-t5.In this case, the red, green and blue LEDs are separately monitored.Thus, the light emitting properties of the LEDs can be obtained withoutperforming a special operation. Thus-obtained emission intensities ofthe red, green and blue LEDs are compared with the reference value. Theresults are fed back to the LEDs to adjust the emission intensities suchthat the difference therebetween becomes zero. Thus, the light emittingdevice 10A can be stable at any white point. As a result of such anadjustment, the emission intensity of the LEDs at or before time t2 andthe emission intensity at or after time t5 are different in the strictsense since they are the values before and after the LEDs receivefeedback.

During the monitoring period t2-t5, the intensity of light entering theeyes is ⅓ of normal. However, since the monitoring period is extremelyshort, for example, 3/200 seconds, the extinction of the light emittingdevice 10A caused by turning off two LEDs can be said to be at a levelwhich is not annoying.

A frequency to monitor the light-emitting property of the LEDs may be,for example, once in one minute. In other words, monitoring periodsmaybe set to have about a one-minute interval. However, in the casewhere the light-emitting property of any of the LEDs changes greatly,the LEDs should be monitored in shorter intervals. On the contrary,while the light-emitting properties of the LEDs indicate a small change,monitoring may be performed in longer intervals.

Second Driving Example of First Embodiment

In FIG. 3 showing the first driving example of the first embodiment,three types of LEDs are turned on one by one in turn by thelight-emission control means 11A during a monitoring period, and, whileone type of LED is turned on, the other two types of LEDs are turnedoff. Thus, there is extinction caused by turning off the two types ofLEDs during a monitoring period, i.e. a decrease in an amount of lightemitted from the light source unit 1, although it is a short period oftime. One of the monitoring methods which avoids an influence of suchextinction is the second driving example of the first embodiment. Inthis driving example, light-emission control means 11B, which is anotherexample of the light-emission control means 11, turns on two of thethree types of LEDs in turn at a time during the monitoring period and,while the two types of LEDs are turned on, the remaining one type of LEDis turned off.

FIG. 4( a)-(c) shows a monitoring method in which two of the three typesof LEDs are turned on in different combinations, in turn, during amonitoring period (in other words, one LED is turned off in turn duringa monitoring period), FIG. 4( a)-(c) respectively indicates the emissionintensity of the red LED, the emission intensity of the green LED, andthe emission intensity of the blue LED. The vertical axes indicatesemission intensity, and the horizontal axes indicates time.

In FIG. 4( a)-(c), during a period from time t1 to t2, all the red,green and blue LEDs are turned on. Thus, the light emitting device 10Aemits white light. Then, a monitoring period starts at time t2. Only thered LED is turned off, and the green and blue LEDs remain in a turned-onstate. As a result, light emitting device 10A emits cyan light. After1/200 second has elapsed from time t2 it becomes time t3, and the redand blue LEDs are turned on, and the green LED is turned off. Thus, thelight emitting device 10A emits magenta light. After another 1/200second has elapsed: it becomes time t4, and the red and green LEDs areturned on, and the blue LED is turned off. Thus, the light emittingdevice 10A emits yellow light. Then, after another 1/200 second haselapsed it becomes time t5, and the monitoring period ends. Three typesof LEDs are all turned on and the light emitting device 10A provideswhite light.

As described above, in the case shown in FIG. 4( a)-(c), only one typeof LED is turned off in turn during the monitoring period. The intensityof light which enters the eyes during this period is ⅔, the degree ofextinction is improved compared to the case shown in FIG. 3. If theemission intensity of the red LED is r, the emission intensity of thegreen LED is 9, and the emission intensity of the blue LED is b, threevalues, i.e., g+b, r+b, and r+g, are obtained for every monitoringperiod. The values r, g and b can be calculated from these values andcompared with the reference value. The results are fed back to the LEDsto adjust the emission intensities such that the difference therebetweenbecomes zero. Thus, the light emitting device 10A can be stable at anywhite point. As a result, the emission intensity of the LEDs at orbefore time t2 and the emission intensity at or after time t5 of theLEDs in FIG. 4( a)-(c) are different in the strictest sense since theyare the values before and after the LEDs receives a feedback.

During the monitoring period t2-t5, the intensity of light which entersthe eyes is ⅔, However, since the monitoring period is extremely short,for example, 3/200 of a second, extinction of the light emitting device10A caused by turning off one type of the LED can be recognized to bealmost at a level which is not annoying.

In the case shown in FIG. 4, a frequency to monitor the light-emittingproperty of the LEDs may be, for example, once in ten seconds. In otherwords, monitoring periods may be set to have about ten second interval.However, in the case where the light-emitting property of any of theLEDs changes greatly, the LEDs should be monitored in shorter intervals.On the contrary, while the light-emitting properties of the LEDsindicate a small change, monitoring may be performed in longerintervals.

In the case shown in FIG. 4, one type of the red, green and blue LEDsmay be turned off in any order. Further, it is not necessary that threetypes of LEDs are turned off one by one in turn. Only one type of LEDcan be turned off during one monitoring period, and all the LEDs areturned off in turn over three monitoring periods.

For further reducing an influence of extinction caused by turning offthe LEDs during the monitoring period from the example described withreference to FIG. 4, monitoring of emission intensities of the LEDs maybe performed when an entire display screen becomes dark rather than at apredetermined interval. In usual television broadcasting, this can beimplemented by utilizing the fact that a nearly black display statetends to appear during transitions between commercial films. In thiscase, a monitoring period starts when the luminance signal among thevideo signals input to the liquid crystal panel 5 has a level near theblack level. Emission intensities of one type or two types of LEDs aremonitored. Even if one type or two types of LEDs are turned off formonitoring the LED, there is substantially no influence of extinctioncaused by turning off the LEDs because the liquid crystal panel 5 isdisplaying a dark screen.

Third Driving Example of First Embodiment

It is also possible to eliminate the influence of extinction caused byturning off the LEDs during a monitoring period in the first and thesecond driving examples of the first embodiment. This method iseffective when there is no image which is nearly black. As describedabove, in the method of the second driving example of the firstembodiment which is described with reference to FIG. 4, two types amongthree types of LEDs are turned on and emission intensities of cyan,magenta, and yellow light are monitored by the optical sensor 4. Thus,the emission intensity of the light emitting device 10A during amonitoring period is ⅔. In a third driving example of the firstembodiment, light-emission control means 11C, which is yet anotherexample of the light-emission control means 11, is set with a thresholdvalue determined from an image signal to display white light. When thelevel of a luminance signal included in video signals is equal to orlower than the threshold value, a monitoring period for monitoringemission intensities of the LEDs is started and the size of a drivingsignal of the liquid crystal panel is extended during the monitoringperiod. Hereinafter, the method is described with-reference to FIG. 5(a)-(d).

In FIG. 5, the vertical axes indicate tone levels of the luminancesignal and horizontal axes indicate a frequency of generation of theluminance signal. As described above, a value 170, which is ⅔ of thevalue corresponding to the white level, 255, is set as a thresholdvalue. At a certain point, if it is detected that level 150, which issmaller than the threshold 170, is a maximum level of the luminancesignal of a certain image, the level of the luminance signal of theimage is distributed between 0 and 150 as shown in FIG. 5( a). Themonitoring period starts at this point, and one type of LED is turnedoff for monitoring the emission intensity of the LED; The emissionintensity of the light emitting device 10A is about ⅔ since the light isemitted from the other two types of LEDs. Therefore, as shown in FIG. 5(b), the level of the luminance signal decreases from 150 to 100 inappearance. In order to avoid extinction of the light emitting device10A by this, a driving signal of the liquid crystal panel 5 can beextended to cancel a decrease in the emission intensity caused byturning of f a LED during the monitoring period over a period duringwhich one type of the LEDs is turned off.

More specifically, in order to avoid extinction of the light emittingdevice 10A, the image should be displayed as if the maximum level is 150over a period in which one type of LED is turned off. Thus, as shown inFIG. 5( c), the size of the driving signal of the liquid crystal panel 5is set to 225, which is a value obtained by multiplying 150 by 3/2. Thisoperation cancels the decrease in the emission intensity of the lightemitting device 10A to ⅔, by multiplying the size of the driving signalof the liquid crystal panel 5 by 3/2. The brightness of the lightemitting device 10A as a result does not experience any change as shownin FIG. 5( c). By compensating the extinction of the light emittingdevice 10A by extending the size of the driving signal of the liquidcrystal panel 5, the influence of the liquid crystal panel 5 can beeliminated. As a result of the actual experimentation there is no changeobserved in appearance.

In the above description, one type of LED is turned on. The similareffect can be obtained in the case when intensities of red, green, andblue light are monitored while two types of LEDs are turned of f at thesame time. However, in this case, the emission-intensity of the lightemitting 10 device 10A is about ⅓. Thus, in the third driving exampleshown in FIG. 5, the threshold valid for determining a time to start themonitoring period is 85, which corresponds to ⅓ of the white levelvalue, 255. In order to eliminate such extinction, the size of thedriving signal of the liquid crystal panel 5 should be extended by threetimes.

In practice, there may be a case where white light is displayed with theluminance signal having the level of 235 or higher. Thus, the thresholdvalues for determining 20 the time to start a monitoring period has tobe determined with a coefficient of gamma correction, or extinction dueto taking the turning off of the LEDs into consideration.

First Monitoring Method of Second Embodiment

In the first monitoring method of the second embodiment, light emittingand turning, off operations which sequentially shift light-emittingtiming of multiple types of tight source during a monitoring period isperformed by, the red, green and blue LEDs. In this case, the emissionintensities of the light sources are made to zero during a turning offoperation.

With reference to FIG. 6, the second embodiment of the light emittingdevice according to the present invention will be described. In thefigure, a light-emitting device 10B includes: a light source unit 1Bprovided with at least one (in the figure, three) light-emitting source,which is a set of a plurality of light sources 2 a, 2 b, and 2 c; alight guide plate 3 for uniformly irradiating a plane with light fromthe light source unit 1B; an optical sensor 4 as a light detection meansfor monitoring the intensity of light transmitted through the lightguide plate 3; and a light emission control means 12 which receivesemission intensity information of the light sources obtained byperforming light emission control of the three types of the lightsources for monitoring during al monitoring period as monitoring resultsfrom the optical sensor 4, and performs light emission control of thethree types of the light sources so as to have a predetermined emissionintensity based on the emission intensity information. The opticalsensor 4 may also be located on an upper portion or a lower portion ofthe light guide plate 3, or at an appropriate position near the lightsource unit 13, not only at the position opposing the light source unit1B with respect to the light guide plate 3 as shown FIG. 6. In thefigure, for facilitating understanding, the components are illustratedto be separate from each other. The differences in the size of thecomponents are emphasized for facilitating understanding, and the actualsizes of the components are different to that illustrated. Further, onlythe minimum components required for understanding the present inventionare illustrated. For example, a light mixing part may be providedbetween the light source unit 1B and the light guide plate 3 forreducing unevenness of light from the light source 2 a-2 c.

In the second embodiment shown in FIG. 6, LEDs of red, green and blue,i.e., the three primary colors of light, are used as a plurality oflight sources in the light-emitting source. The light emitted from theLEDs are mixed with each other and become generally white light. Thelight passes the light guide plate 3 and emits in a direction indicatedby the arrow shown In FIG. 6. Thus, the light emitting device 10B isformed. A liquid crystal panel (not shown) is located such that itreceives the light emitted from the light guide plate 3 to form a liquidcrystal display apparatus. Further, the direction to emit lightindicated by the arrow in FIG. 6 can be controlled by a surfacestructure of the light guide plate 3.

It is desirable to provide a reflection plate such as an aluminum mirroron a side surface of the light guide plate 3 in order to effectivelyemit light from the light guide plate 3 to the exterior. The light fromthe light source unit 1 must reach the optical sensor 4 via the lightguide plate 3. Thus, it is necessary that the reflection plate is notprovided on a portion of the light guide plate 3 to which the opticalsensor 4 opposes, or a reflecting part which slightly passes light isprovided on that portion.

FIGS. 7( a), (b), (c) and (d) shows the first monitoring method formonitoring an operation of a light source when pulse-width control oflight emitted from the red, green, and blue light sources in onelight-emitting source of the light source unit 1B of FIG. 6 isperformed. In the figure, horizontal axes indicate time, and verticalaxes indicate current values (or emission intensities). Herein, lightemission control means 12A, which is an example of the light emissioncontrol means 12, perform the pulse width control of the light sources.Thus, for example, the red light source emits light from time t1 to t4as shown in FIG. 7( a), the green light source emits light from time t2to t5 as shown in FIG. 7( b), and the blue light source emits light fromtime t3 to t6 as shown in FIG. 7( c). As a result, the emissionintensity as a whole light-emitting source changes in a step-wise mannerover time as shown in FIG. 7( d). Specifically, during the period fromtime t1 to t2, the emission intensity is that of only the red lightsource. During the period from time t2 to t3, the emission intensity isthat caused by the simultaneous operation of the red light source andthe green light source. During the period from time t3 to t4, theemission intensity is that caused by the simultaneous operation of thered light source, green light source, and blue light source, i.e., theemission intensity of the entire light-emitting source.

Light-emitting operations of, the light sources are controlled by apulse driving circuit, Thus, it is already known which of the lightsources is emitting light during a certain period of time. Therefore,when a change in the light sources is monitored in an interval of shortamount of time by the optical sensor 4, the emission intensities inappearance of the light sources can be obtained un-ambiguously.Specifically, the emission intensity during the period from time t1 tot2 is that of the red-light source. Thus, if the emission intensity ofthe period from time t1 to t2 is subtracted from the emission intensityin the period from time t2 to t3, the emission intensity of the greenlight source can be obtained. Similarly, if the emission intensity fromtime t2 to t3 is subtracted from the emission intensity of the periodfrom time t3 to t4, the emission intensity of the blue light source canbe obtained. This is because the apparent emission intensity is obtainedthrough integral of the emission intensity to time. Based on theemission intensity obtained in this way, an emission intensity which isstable in appearance can be obtained by appropriately adjusting theemission intensities and light-emitting times of the light sources evenwhen the emission intensities of the, light sources change due to atemperature change or a change over time.

Adjusting the emission intensities and light-emitting time of the lightsources may be implemented by, for example, making a deviation obtainedby comparing the output of the optical sensor 4 and the predeterminedset value zero, i.e., controlling the light emitting operations of thelight sources so as to match the set value. Matching to the set valuemay be performed by, for example, the algorithm described below. Asdescribed above, the emission intensities in appearance of the lightsources correspond to the emission intensities of the light sourcesintegrated by light-emitting time. Actually, the light-emitting time isextremely short. Thus, it is possible to regard that the emissionintensity does not change during this period. Therefore, the apparentemission intensity can be obtained as a product of the light-emissionintensity and the light emitting time. An output from the optical sensor4 and the predefined set value are compared to obtain the differencebetween them. When the obtained difference has a positive value, theemission intensity in appearance is strong. Thus, the light-emittingtime of the light source is controlled to be shorter. On the other hand,when the obtained difference has a negative value, the emissionintensity in appearance is weak. Thus, the light-emitting time iscontrolled to be longer. Such a control is performed in a subsequent fewcycles to adjust the light-emitting time such that the differencebetween the emission intensity and the set value become zero for each ofthe light sources. By matching the respective emission intensities ofthe light sources to the set value, it becomes possible to controlluminance and chromaticity.

An algorithm for matching the emission intensity to the set value is notlimited to the above example. Instead, a ratio of the output of theoptical sensor 4 and the set value may be taken to control the emissionintensity. It is also possible to store the light-emitting timedetermined as a result of a luminance adjustment and/or chromaticityadjustment by the user and to perform control using the storedlight-emitting time as the set value to stably maintain the luminanceand/or chromaticity adjusted by the user.

In the second embodiment for monitoring the emission intensities, asshown in FIG. 6, fewer optical sensor(s) 4 fewer than the number oflight sources, for example, one optical sensor in the case of FIG. 6, isused by sequentially shifting the timing for the respective lightsources to emit light in order to allow the red, green and blue lightsources to perform light-emitting operations In the first monitoringmethod shown in FIG. 7 by the light emission control means 12A. In thiscase, the monitoring time during which the light sources are turned onand off in turn (for example, a period from time t1 to t3 in FIG. 6) isextremely short and cannot be detected by the eye. A frequency toperform such monitoring is arbitrary, but it is desirable to performfrequently when a change in the emission intensity is large, such as,when power is turned on.

The order to monitor a plurality of light sources during one monitoringperiod is arbitrary, and not limited to the above-mentioned order ofred, green, and blue. Further, it is not necessary to monitor theemission intensities of all the light sources with in one monitoringperiod. The light sources fewer than all the light sources may bemonitored in one monitoring period, and the emission intensities ofmultiple types of light sources may be calculated after a plurality ofmonitoring periods.

For example, when an LED driver of a switching method (DC/DC converteror chopper) is used, as the light-emitting control means 12, there ismore noise than in the case of a LED driver utilizing a current limitingresistance or a constant current load (series regulator). Thus, a colorhaving longer light-emitting time (color with a large PWM wave duty) maybe turned on by priority. In this way, it is possible to enter the nextmeasuring cycle after a long time has elapsed after the light sourcesare turned off and the noise of the power supply line becomes steady.

It is not necessary that monitoring of the emission intensities of thelight sources be performed by shifting the timings for the light-sourcesto emit light. Instead, as indicated in FIG. 7( d) as time t4, t5, andt6, timing to turn off the light sources may be slightly shifted toperform the monitoring. This is possible because the period for thelight sources to emit light can be previously set and is also determinedby the result of monitoring by the optical sensor 4, and thus, thetiming to turn off the light sources can be shifted. This, small shiftis utilized to monitor the emission intensities.

The amount of light may be further monitored in the state where all thelight sources are-turned off (a period from t6 to t7 when the lightsource emits light in FIG. 7). This allows a more accurate control whenthe sensor value does not become zero due to an influence such asoutside light by using this value (monitored result) as a background andcalculating the emission intensities from a difference between thisvalue and the measured values. Further, not only the influence of theoutside light but also the influence of a dark current (the currentgenerated even when the amount of received light is originally zero) canbe suppressed.

In the second embodiment shown in FIG. 6, the light source unit 1B islocated on a side surface of the light guide plate 3. However, thelocation or the shape of the light source unit 1B is not limited tothis. For example, the light source unit 1B may be located on a backsurface of the light guide plate 3, and light can be expanded andprojected therefrom. Further, in the first embodiment, the light sourcesof the three primary colors, red, green and blue are combined to producecomposite white light. However, the light sources of two colors, blueand yellow can be used to form a light source unit 1B to monitoremission intensities of the two light sources. Moreover, the opticalsensor 4 may be located at any position as described above. However, aplurality of optical sensors of the same type may be provided. Eventhough a plurality of the optical sensors are provided, it isadvantageous in view of cost because they are of the same type, and italso becomes possible to monitor variances in luminance and/orchromaticity by using a plurality of optical sensors.

Second Monitoring Method of Second Embodiment

In the second embodiment, the red, green, and blue light sources performlight-emitting operations and turning off operations to sequentiallyshift the timing to emit light during monitoring. Particularly, in thesecond monitoring method, the emission intensities of the light sourcesare not zero but have predetermined emission intensities during theturning off operation. In this case, light emission control means 12B,which is another example of the light emission control means 12,performs switching control between the first emission intensity and thesecond emission intensity which is lower than the first emissionintensity.

Specifically, in the description with respect to the first to thirddriving examples of the first embodiment and the first monitoring methodof the second embodiment, the emission intensities of the light sourcesare made to be zero in turn during the monitoring period for monitoringthe light emission intensities. However, the emission intensities arenot necessarily zero. This is particularly effective for a light sourcewhich has persistence, such as an LED using a phosphor and a coldcathode fluorescent tube. FIGS. 8( a), (b), (c) and (a) is a diagramillustrating the second monitoring method for monitoring the emissionintensities of the light sources of which the emission intensities donot become zero when they are turned off. The horizontal axes indicatetime and the vertical axes indicate emission intensity of the lightsources.

The light emitting operations of the light sources are as follow. Asshown in FIG. 8( a), the red light source starts to emit light atintensity a at time t1 and attenuates light to intensity a at time t4during the first cycle, starts to emit light at intensity a at time t7and attenuates light to intensity a cat time t10 during the secondcycle, and starts to emit light at intensity a at time t14 andattenuates light to intensity a at time tl7 during the third cycle.

Similarly, as shown in FIG. 8( b), the green light source starts to emitlight at intensity b at time t2 and attenuates light to intensity β attime t5 during the first cycle, starts to emit light at intensity b attime t9 and attenuates light to intensity β at time t12 during thesecond cycle, and starts to emit light at intensity b at time t15 andattenuates light to intensity β at time t1 during the third cycle.

As shown In FIG. 8( c), the blue light source similarly starts to emitlight at intensity c at time t3 and attenuates light to intensity γ attime t6 during the first cycle, starts to emit light at intensity c attime t8 and attenuates light to intensity γ at time t11 during thesecond cycle, and starts to emit light at intensity c at time t13 andattenuates light to intensity γ at time t16 during the third cycle.

Since the red, green and blue light sources emit and attenuate light asdescribed above, the emission intensity of the light emitting sourceformed of such light sources experiences a change as shown in FIG. 8(d), which Includes increases and decreases in a step-wise manner.Herein, the period during which the emission intensity increases in astep-wise manner is a monitoring period. Intervals within the monitoringperiod which have different emission intensities are referred to as thefirst step, the second step, and the third step in ascending order oftheir emission intensities. For example, in FIG. 8( d): in the firstcycle, the interval from time t1 to t2 is the first step, the intervalfrom time t2 to t3 is the second step, and the interval from time t3 tot4 is the third step; in the second cycle, the interval from time t7 tot8 is the first step, the interval from time t8 to t9 is the secondstep, and the interval from time t9 to t10 is the third step: and in thethird cycle, the interval from time t13 to t14 is the first step, theinterval from time t14 to t15 is the second step, and the interval fromtime t15 to t16 is the third step. The following table, Table 1, showsthe values of the emission intensities in the first to the third stepsin the first to the third cycles.

TABLE 1 First cycle Second cycle Third cycle First step a + β + γ a +β + γ α + β + c Second step a + b + γ a + β + c a + β + c Third step a +b + c a + b + c a + b + c

Table 1 contains six variables, a, b, c, α, β and γ. The six variablescan be obtained by using six values in total, for example, three valuesof the first to third steps in the first cycle, two values of the firstand second steps in the second cycle, and one value of the first step ofthe third cycle. The emission intensities of the light sources when thelight is emitted or attenuated obtained as such are used to adjust theluminance and/or chromaticity.

In the monitoring method described with reference to FIG. 8( a) to (a),the light sources emit light at different emission intensities in eachof the first to third cycles. These three cycles are combined into onebig cycle for obtaining the emission intensities of the light sources.Such a method is different on the point that monitoring is completedwith one cycle including a plurality of monitoring periods from themonitoring method which has been already described with reference toFIG. 7, in which monitoring is completed within one monitoring periodconsisting of three sequential intervals of a short period of time. Thisdifference is merely a difference in setting points to start and finishmonitoring, and there is no substantial difference in the effect ofcontrolling the emission intensities.

In the monitoring method of FIG. 8, the red, green, and blue lightsources can emit light in an arbitrary order and at arbitrary timing. Aslong as the timings to become emission intensities. a, b, and c do notoverlap, the order may not necessarily be the one as shown in FIG. 8.

Third Monitoring Method of Second Embodiment

Multiple types of light sources in the light emitting device shown inFIG. 6 are controlled by pulse width control as shown in FIG. 7 (firstmonitoring method) or FIG. 8 (second monitoring method). In the thirdmonitoring method, light emission control means 12C, which is furtheranother example of the light emission control means 12, may drive themultiple types of the light sources by current value control. In thiscase, the light sources independently attenuate light for a very shorttime period for monitoring the emission intensities of the lightsources. The light-emitting operations of the light sources in such acase is shown in FIGS. 9( a), (b), (a) and (d). The horizontal axesindicate time, and the vertical axes indicate emission intensity(current values) of the light sources.

Specifically, as shown in FIG. 9( a), the red light source, normallyemits light at intensity a from time t1 to t2, emits attenuated light atintensity a from time t2 to t3, again emits light at intensity a fromtime t3 to t5, emits light at intensity a from time t5 to t7, and emitslight at intensity a at time t7 and after.

Similarly, as shown in FIG. 9( b), the green light source normally emitslight at intensity b from time t1 to t3, emits attenuated light atintensity β from time t3 to t4, emits light at intensity b from time t4to t5, emits light at intensity β from time t5 to t6, emits light atintensity b from time t6 to t7, emits attenuated light at intensity βfrom time t7 to t8, and emits light at intensity b at time t8 and after.

As shown in FIG. 9( c), the blue light source normally emits light atintensity G from time t1 to t4, emits attenuated light at intensity γfrom time t4 to t5, again emits light at intensity c from time t5 to t6,emits attenuated light at intensity γ from time t6 to t8, and emitslight at intensity c at time t8 and after.

The emission intensity of the entire light-emitting source in theabove-described operation varies as shown in Table 2 below from time t1to t8 as indicated in FIG. 9( d).

TABLE 2 Time Emission intensity From t1 to t2 a + b + c From t2 to t3α + b + c From t3 to t4 a + β + c From t4 to t5 a + b + γ From t5 to t6α + β + c From t6 to t7 α + b + γ From t7 to t8 a + β + γ

Among the emission intensities shown in Table 2, by solving simultaneousequations for six values from time t2 to t8, values or the six variablesa, b, c, α, β and γ can be obtained. By obtaining emission intensitiesof the optical sources, adjustment of white point and/or luminance canbe performed as described above with reference to FIGS. 7 and 8.However, for controlling the emission intensity by controlling thecurrent values, it is not necessary to take the integral of the emissionwith respect to the light-emitting time. As described above, theapparent emission intensity indicates the emission intensity.

In the monitoring method as shown in FIG. 9, the light sources can emitlight in any order as long as there is a period when one light sourceattenuates light and a period when the other two light sources attenuatelight. For example, in the case where three types of light sources areused as shown in FIG. 9, as long as there are six types of extinctionstates, their order and timing can be arbitrary. With reference to FIG.9, it is described that the light sources attenuate lights in a periodfrom time t2 to t8. However, the light sources may be controlled toincrease the intensities of light.

In the case where values for three variables, α, β and γ are zero, inother words, three light sources are turned off, there are threevariables, a, b and c. Thus, it is sufficient if three different statesare provided during one monitoring period. This is as described abovewith reference to FIGS. 3 and 4.

Third Embodiment

FIG. 10 schematically shows a light emitting device 10C of the thirdembodiment according to the present invention. In the third embodiment,the light emitting device 10C includes: a light source unit 1C providedwith a plurality of light-emitting sources, comprising two types oflight sources 2 a and 2 c; a light guide plate 3 for uniformlyirradiating a plane with light from the light source unit 1C; a secondlight source unit 6 including a light source 2 b of a type differentfrom the above light sources; a light guide plate 7 for uniformlyirradiating a plane with light from the second light source unit 6; anoptical sensor 4 as a light detection means; and light emission controlmeans 11 or 12 which receives emission intensity information of thelight sources obtained by performing light emission control of the threetypes of the light sources for monitoring during a monitoring period asmonitoring results from the optical sensor 4, and performs lightemission control of the three types of the light sources so as to have apredetermined emission intensity based on the emission intensityinformation. The optical sensor 4 for monitoring intensity of lighttransmitted through two light guide plates 3 and 7: is provided on thecenter of the two light guide plates 3 and 7 upper portions such thatthe optical sensor 4 bridges over the light guide plates 3 and 7. Thus,the optical sensor 4 receives light equally from two light guide plates3 and 7.

In the third embodiment, the components are separately illustrated andthe sizes of the components are different from the actual sizes.Further, it should be noted that FIG. 10 shows only the minimumcomponents required for description. For example, a light mixing partmay be provided between the first light source unit 1C and the lightguide plate 3 and/or between the second light source unit 6 and thelight guide plate 7 in order to reduce the color unevenness of lightfrom multiple types of light sources 2 a, 2 b and 2 c.

One optical sensor 4 is provided as described above, for the sake ofreducing cost. If there is no problem in terms of cost, one opticalsensor can be provided for each of the light guide plates 3 and 7. Inthe case of providing one optical sensor 4, it is not necessary that theoptical sensor 4 is provided in the center of the upper portions of thelight guide plates 3 and 7. The optical sensor 4 may lean to either thelight guide plate 3 or 7. Further, the optical sensor 4 may be providedon lower portions instead of upper portions as shown in FIG. 10. Inshort, the optical sensor 4 may be fixed to any position as long as sucha state can be defined as an initial state and the emission intensitiesof the light sources can be adjusted.

In the light-emitting device 10C of FIG. 10, for example, the lightsource 2 a is a red LED, the light source 2 b is a green LED, and thelight source 2 c is a blue LED. In the first light source unit 1C, redand blue LEDs are provided, and, in the second light source unit 6,green LEDs are provided. Light emitted from the LEDs passes the lightguide plates 3 and 7, and emits in a direction indicated by the arrow inthe figure. Use of two light guide plates as described above allows thelight sources to be located on both sides, and thus it is effective inenhancing the intensities of light.

It is also possible to locate light-emitting sources comprising red,green and blue LEDs on both sides of the light guide plates. However, inview of the emission efficiency of the current state, it is appropriateto provide LEDs such that their ratio in numbers among colors is 1:2:1for emission adjustment in order to reproduce white light from threecolors, red, green and blue. Taking this into account, to locate red andblue LEDs on one side and green LEDs on the other side as shown in FIG.10 has a big merit. The reason for this is described below.

In the case where the red, green and blue light sources are located onone side of the light guide plate, since the emission intensity detectedby the optical sensor is the sum of the light from the light sources onone side of the light guide plate, the sum of the emission intensitiesfor each of the colors can be obtained but the emission intensity ofeach of the light sources cannot be obtained as it is. Therefore, forindividually adjusting the emission intensities of the light sources onone side, any of the monitoring methods described with reference toFIGS. 7-9 should be performed for the light sources on both sides, i.e.,should be repeated twice. On the other hand, in the case where the redand blue light sources are provided on one side of the light guide plateand the green light sources are provided on the other side of the lightguide plate, the emission intensities of the light sources can beobtained by performing any of the monitoring methods described withreference to FIGS. 7-9 only once. Although the current values flowingthrough the light sources can be recognized for a certain degree, as itis impossible to precisely grasp the changes including changes of thelight source over time, changes in the states due to heat generation,and the like, monitoring method for monitoring the emission intensitiesof the light sources and the observations feeding back have atechnically significant meaning.

A display apparatus is formed by locating a liquid crystal panel infront of the light emitting device 10B or 10C as shown in FIG. 6 or 10.Light having the adjusted emission intensity passes through the liquidcrystal panel and displays characters and images. The light-emittingdevice may be placed behind the liquid crystal panel to be used as abacklight, or may be located in front of the reflective type liquidcrystal panel to be used as a front light.

In the case where the light emitting device 10B or 10C is used as afront light of the reflective type liquid crystal panel, if the valuesof α, β and γ are equal to or greater than the threshold values, it isdetermined that outside light (ambient light, illuminance of ambientcircumstance) is sufficient and the LEDs of the lights sources may becompletely turned off. In the case where the light emitting device 10Bor 10C is employed in a display of a digital camera, or a mobile phonewith a built-in camera, the optical sensor of the present invention maybe applied for determining whether to use a strobe light or aflashlight. This is because the optical sensor and peripheral circuitsare originally designed with high precision such that they can also beused for photometry, and thus, they can be used as an optical sensor forcomparing with the threshold values, such as infrared remote control,obstruction detection, determination of sunset, or the like.

In a studio for recording a TV program, amusement facility or the like,one large display apparatus, which is formed by combining a plurality ofrelatively small display apparatuses, may be used. For example, if 16 of30-type displays are arranged into four rows and four columns, one120-type display can be implemented. In this case an optical sensor maybe provided in each of the small display apparatuses. The presentinvention is effective for absorbing individual differences among thedisplay apparatuses in a so-called multi-monitoring system.

In the liquid crystal display apparatus which has a screen size of 30 or40, a plurality of small backlight units may be arranged to form oneplane light source for simplifying assembly, maintenance, or the like.In such a case, a sensor may be provided for each of the backlightunits. Even though heat radiating conditions in the units provided onthe lower side and those in the units provided on the upper side do notmatch due to the influence of the gravitational field of the earth, airconvection or the like, the sensors absorbs such differences. Thus, itis not necessary to be careful about thermal design, place ofinstallment; or the like.

Fourth Embodiment

The light emitting device 10A. 10B, and 10C which has been describedabove can be applied to a read apparatus. In the fourth embodiment, theabove-described light emitting device 10A, 10B, or 10C is applied to aread apparatus.

FIG. 11 shows an example; (a) schematically shows a read apparatus, and(b) schematically shows the light emitting device according to thepresent invention.

As shown in FIG. 11( a), the read apparatus 11 includes: a read portion8 which operates as a scanner, copying machine or the like; a read copyholder 9 as a stage for putting a copy to be read, and a light-emittingdevice 10 for illuminating the copy.

As shown in FIG. 11( b), the light-emitting device 10 is formed of alight emitting portion 10 a for emitting light so as to uniformlyilluminate the copy, and a light source unit 10 b in which multipletypes of light sources are located. The light source unit 10 bincorporates red, green and blue light sources and an optical sensor(not shown) for monitoring emission intensities of these light sources.When the red, green, and blue LEDs are used as light sources, anillumination with more vivid colors compared to a cold cathodefluorescent tube or white LED can be implemented. A copy placed on theread copy holder 9 is illuminated with light from the light-emittingdevice 10 having the above-described structure, reflects the light withvivid colors, and is read in the read portion 8. For adjusting theemission intensities of the light sources in the light source unit 10 b,any of the monitoring methods described with reference to FIGS. 7-9 maybe used.

Among the optical sensors, an optical sensor for controlling luminanceand chromaticity and a licensor for reading a copy may be of the sametype. It may be needless to say that operations must be controlled in atime-divisional manner so that the operations do not conflict.

Currently, a photocell, a photo-multiplier, a photodiode, and the likeare known as an optical sensor element so suitable for photometryapplications. Hereinafter, the characteristics of these elements will bedescribed.

In a photocell which is sensitive to visible light, CdS (cadmiumsulfide) is used. If a photocell is employed, it becomes difficult touse in view of the low degree of environmental load, compared to a CRT(cathode ray tube) using lead glass, or a CCFL (cold cathode fluorescentlamp) using mercury. If an obligation to recycle products using cadmiumexists in the future, the cost will rise. There is also a possibilitythat use of cadmium itself will be banned.

A photo-multiplier has too-large a scale for this application. Not onlyinexpensive cost, but also in that the ease of maintenance is at a lowlevel.

The other element is a photodiode. This can be divided into severalgroups depending on the materials. Amorphous silicon photodiodes showspectral sensitivity characteristics similar to the luminosity factor ofa human. However, the mobility of a carrier in a semiconductor is smalland the response speed is slow Thus, it is difficult to use a photodiodefor the purpose of the present invention. On the other hand, a singlecrystal silicon photodiode does not have a problem of a response speed,but has a defect that it has sensitivity to infrared radiation.

In the present invention, it is sufficient if outputs of red, green, andblue lamps are controlled at constant levels. Thus, generally, there isno problem even if the spectral sensitivity of an optical sensor issomewhat different from the luminosity factor of a human. It is ratherpreferable that the spectral sensitivity characteristics are flatbecause an S/N ratio (signal to noise ratio) is higher.

In the case where LEDs are employed for lamps as light sources, thespectral sensitivity characteristics of the optical sensor from red toinfrared radiation cannot be ignored. This is because AlGaInP (aluminumgallium indium phosphide) type red LED is more sensitive to temperaturechange in a junction than green or blue LEDs of GaInN (gallium indiumnitride), and has unstable luminance and also emission wavelength. Inother words, the emission wavelength becomes longer as the temperatureincreases. This wavelength shift is so large that it cannot bedisregarded in this application.

Even though the temperature at the junction increases, for obtaining anoutput proportional to the luminance, the spectral sensitivity of theoptical sensor has to match the luminosity factor characteristics of ahuman. Thus, a luminosity factor filter is inserted between a lightguide plate and the optical sensor to block the infrared radiation. Asshown in FIG. 14, the spectral sensitivity from the red light to theinfrared radiation should match the luminosity factor. Thus, even if theemission frequency of the red light changes due to self heat generation,a change in ambient temperature, or the like, the optical sensor cantrack the change. In other words, even if the wavelength becomes longer,the gain of the sensor can be decreased In proportion to the luminosityfactor of a human.

FIG. 14 is a graph depicting a portion of concern for the sake ofunderstanding. Actually, it is sufficient if the spectral sensitivity ofthe optical sensor approximately matches the luminosity factor of ahuman, in the vicinity of the emission wavelength of the red LED.

It is also found that an effect of feed back control of the presentinvention changes due to the spectral sensitivity of the sensor fromread light to infrared radiation, and thus, the light emitting devicewhich handles this is added. It is optimum to adjust the spectralsensitivity of the optical sensor to the luminosity factor of a humanwith the emission wavelength of the AlGaInP type red LED. FIG. 14 is agraph for illustrating this.

There are a variety of luminosity factor filters on the points of price,transmittance of light (sensitivity of the sensor), resistance toenvironment (temperature under burning or scorching, temperature atsoldering for mounting, or the like), and other properties due to degreeof precision with which they are produced. It is needless to say thatthe temperature characteristic of a luminosity factor has to besufficiently smaller than the temperature characteristic of the LEDs.For a display apparatus used for applications such as a televisionreceiver, word processor, terminal device for e-mail, technical drawing,or the like, it is much more important that stability is high andmaintenance is not necessary rather than pursuing high precision.

It is confirmed by experimentation that, if a material is selected withattention to the spectral sensitivity characteristics, the presentinvention provides sufficient characteristics in practical use. FIG. 15shows the results actually measured by using two types of sensors.

Without feedback control of the present invention (without feedback),the relative luminance after the backlight is lit increases by about25%. This can be perceived easily and it is beyond the tolerance limit.In the case where a sensor with sensitivity to infrared radiation, whichdoes not have a luminosity factor filter, is used, a change in luminanceis improved to about 10%. In the case where infrared radiation isblocked by the luminosity factor filter, a change in luminance issuppressed to 4%. Accordingly, if the spectral sensitivity of theoptical sensor is taken into consideration, the luminance can bestabilized at a speed faster than not only a CRT but also a CCFL. Asdescribed above, a specific effect of the feedback control of thepresent invention (FIG. 15) was confirmed by experimentation.

The fourth embodiment of a light emitting device, and a displayapparatus and a read apparatus using the light emitting device as anauxiliary light source has been described above. However, the presentinvention is not limited to the first through fourth embodiments.Hereinafter, variations of the first through fourth embodiments of thepresent invention will be listed.

(1) Regarding light source, any light source may be used instead of theLEDs. However, in the present invention, the light sources are turned onand off in short time. Thus, a light source which can-be driven at afast rate such as an LED is preferable.

(2) The light-emitting device shown in FIGS. 1 and 2 emits white light.Thus, the light source unit 1 includes light sources which emit light ofred, green and blue colors. However, the number and the types of thelight sources forming the light source unit 1 may be determineddepending on which of the colors it is desired to be emitted by thelight emitting device. For example, in the light-emitting device foremitting magenta light, red and green light sources are provided in thelight source unit, and the LEDs are turned off in turn one type at atime during a monitoring period.

(3) In FIGS. 1 and 2, the optical sensor 4 is located on the light guideplate 3 so as to oppose the light source unit 1. However, the positionof the optical sensor 4 is not limited to this, and may be located atany position on the guiding plate 3. Further, the optical sensor 4 maybe located on the light source unit 1 or the light mixing part 2.

(4) A time period during which the LEDs are being turned on or off inthe monitoring period is not limited to 1/200 second. An appropriatelength for the period may be selected in accordance with the types andthe number of the light sources.

(5) It is not necessary to feed back the monitoring results by opticalsensor 4 to the light sources in every monitoring period. It is alsopossible to appropriately process the monitoring results over aplurality of subsequent monitoring periods before feeding back toenhance the precision.

(6) The order to drive the multiple types of light sources emittinglight of different colors during one monitoring period is arbitrary,and, not limited to the order of red, green, and blue as describedabove.

(7) It is not necessary to complete monitoring of all the light sourceswithin one monitoring period. Monitoring of one type of light source maybe completed in one monitoring period to complete monitoring of all thelight sources in a plurality of sequential monitoring periods,

(8) The light emitting device means not only an auxiliary light sourcefor a display apparatus or read apparatus but also an illumination lightsource for irradiating a space.

As can be seen from the description of one embodiment of a displaydevice of the present invention, and a display apparatus using thedisplay device as an auxiliary light source, according to the presentinvention there is provided a light emitting device comprising multipletypes of light source emitting light of different colors, whichcomprises light emission control means for allowing at least one lightsource among the multiple light sources to emit light during apredetermined period for monitoring emission intensities at an emissionintensity different from that in the period other than the predeterminedperiod. Thus, the following significant effects are provided.

(1) The emission Intensities of the light sources can be monitored withthe optical sensor(s) of a number fewer than the types of the opticalsources, and a light emitting device without variance can be obtained atlow cost.

(2) Since the emission intensities of the at least one light sourceamong multiple types of light sources are controlled using the resultmonitored during the predetermined period, the light emitting devicewhich can adjust the white point and/or emission intensities can beobtained.

(3) Emission properties of the light sources can be adjusted withoutcausing a substantial influence in appearance during the operatingperiod of the light sources.

(4) A light emitting device using any combination of the light sourcescan be adjusted suitably at an appropriate time. Thus, the lightemitting device can always be operated in a suitable state.

(5) Since the emission intensities of the light sources are controlledby current values or light emitting time, the light emitting devicewhich can readily control the emission intensities can be obtained.

(6) The emission luminance and/or emission chromaticity are controlledto desired values by controlling the emission intensities of the lightsources. Thus, the light emitting device providing stable luminance andchromaticity can be obtained.

(7) By using, for example, LEDs as multiple types of the light sources,the light emitting device having high color purity can be obtained.

(8) By using the light emitting device according to the presentinvention, display apparatus and read apparatus which have controllablewhite point and/or emission intensity can be obtained.

INDUSTRIAL APPLICABILITY

In the field of a light emitting device including light sources whichemit light of multiple colors, display apparatus using the lightemitting device, and a read apparatus using the light emitting device,emission intensities of multiple types of the light sources can bemonitored with fewer types of the optical sensors, and white pointand/or luminance properties can be controlled.

1. A light emitting device comprising: multiple types of light sourcesemitting light of different colors; light detection means for monitoringemission intensity of at least one light source among the multiple typesof light sources; and light emission control means which performscontrol to provide a light emitting period in which all of the multipletypes of light sources emit light at a same time at predeterminedemission intensities and a monitoring period in which an emissionintensity of only a single light source is decreased, wherein the lightemission control means controls the emission intensity of the at leastone light source among the multiple types of light sources usingemission intensity information from the light detection means in themonitoring period to adjust composite light from the multiple types oflight sources to have a desired luminance or chromaticity.
 2. A lightemitting device according to claim 1, wherein the light emission controlmeans provides the monitoring period by shifting one of timing to obtainthe predetermined emission intensities and timing to decrease theemission intensity of the single light source with respect to timing toobtain the predetermined emission intensities and timing to decrease theemission intensity of other single light source.
 3. A light emittingdevice according to claim 1, wherein the light emission control meansdecreases the emission intensities of the at least one of but fewer thanthe number of the multiple types of light sources in the monitoringperiod.
 4. A light emitting device according to claim 3, wherein thelight emission control means provides the monitoring period by shiftingone of timing to obtain the predetermined emission intensities andtiming to decrease the emission intensities of at least one of but fewerthan the number of the multiple types of light sources with respect totiming to turn on or timing to turn off other light sources.
 5. A lightemitting device comprising: multiple types of light sources emittinglight of different colors; light detection means for monitoring emissionintensity of at least one light source among the multiple types of lightsources; and light emission control means which performs control toprovide a light emitting period in which all of the multiple types oflight sources emit light at a same time at predetermined emissionintensities and a monitoring period in which emission intensities of atleast one of but fewer than the number of the multiple types of lightsources are increased to a value greater than zero and greater than thelight-emitting intensity of each of the light sources during a periodduring which all of the types of light sources are made to emit light atthe same time, wherein the light emission control means controls theemission intensity of the at least one light source among the multipletypes of light sources using emission intensity information from thelight detection means in the monitoring period to adjust composite lightfrom the multiple types of light sources to have a desired luminance orchromaticity.
 6. A light emitting device according to claim 5, whereinthe light emission control means provides the monitoring period byshifting one of timing to obtain the predetermined emission intensitiesand timing to increase the emission intensities of at least one of butfewer than the number of the multiple types of light sources withrespect to timing to obtain the predetermined emission intensities andtiming to increase the emission intensities of other light sources.
 7. Alight emitting device according to any one of claims 1 to 6, wherein thelight detection means has spectral sensitivity characteristicsapproximately matching luminosity factor characteristics with a lightemission wavelength of the at least one of the multiple types of lightsources being a center.
 8. A light emitting device according to any oneof claims 1 to 6, wherein the light detection means includes aluminosity factor filter for blocking infrared radiation.
 9. A lightemitting device according to any one of claims 1 to 6, wherein the lightemission control means provides a period in which all of the multipletypes of light sources are turned off, the light detection meansmonitors amount of light in a state that all of the multiple types oflight sources are turned off.
 10. A light emitting device according toclaim 5, wherein the light emission control means increases the emissionintensities of the at least one of but fewer than the number of themultiple types of light sources in the monitoring period.
 11. A lightemitting device according to claim 10, wherein the light emissioncontrol means provides the monitoring period by shifting one of timingto obtain the predetermined emission intensities and timing to increasethe emission intensities of at least one of but fewer than the number ofthe multiple types of light sources with respect to timing to turn on ortiming to turn off other light sources.
 12. A light emitting device,comprising: multiple types of light sources emitting light of differentcolors; light detection means for monitoring emission intensity of atleast one light source among the multiple types of light sources; andlight emission control means which performs control to provide a lightemitting period in which all of the multiple types of light sources emitlight at a same time at predetermined emission intensities and amonitoring period in which at least one of but fewer than the number ofthe multiple types of light sources emits light at emission intensitydifferent from that in the light emitting period in which all of themultiple types of light sources emit light at the same time, wherein thelight emission control means provides a period in which all of themultiple types of light sources are turned off, the light detectionmeans monitors amount of light in a state that all of the multiple typesof light sources are turned off, the light emission control meanscorrects emission intensity information from the light detection meansin the monitoring period based on the state that all of the multipletypes of light sources are turned off.
 13. A light emitting device,comprising: multiple types of light sources emitting light of differentcolors; light detection means for monitoring emission intensity of atleast one light source among the multiple types of light sources; andlight emission control means which performs control to provide a lightemitting period in which all of the multiple types of light sources emitlight at a same time at predetermined emission intensities and amonitoring period in which emission intensities of at least one of butfewer than the number of the multiple types of light sources aredecreased to a value greater than zero and less than the light-emittingintensity of each of the light sources during a period during which allof the types of light sources are made to emit light at the same time,wherein the light emission control means controls the emission intensityof the at least one light source among the multiple types of lightsources using emission intensity information from the light detectionmeans in the monitoring period to adjust composite light from themultiple types of light sources to have a desired luminance orchromaticity.